1. Field of the Invention
The present invention relates generally to a fluorescent lamp, and more particularly to a fluorescent lamp having reduced mercury consumption.
2. Description of Related Art
Mercury vapor discharge fluorescent lamps are well known in the marketplace. Their operation depends upon the excitation of mercury vapor atoms via an electric discharge, and the resonance energy given off when the excited atoms return to their ground state. Each lamp therefore contains a quantity of mercury sufficient to maintain the desired mercury vapor pressure within the sealed lamp (typically 4-6 xcexcm Hg). So long as the mercury vapor within the lamp remains at the desired pressure, the lamp will continue to operate normally, producing maximum lumens.
Unfortunately, mercury vapor is depleted over the lamp""s life via a number of known mechanisms. These mechanisms include reaction with phosphor particles or phosphor additives in the phosphor coating, and reaction with the glass envelope itself. Reaction with the glass envelope is the most significant source of mercury vapor depletion, however both reactions deplete mercury vapor by consuming atomic mercury in a chemical reaction.
One solution to this problem has been to provide an alumina or silica barrier layer on the inner surface of the glass envelope to prevent mercury attack. Alumina and silica have been somewhat successful at abating mercury-glass reactions, however it is known that yttria is more effective than either alumina or silica for this purpose. Yttria barrier layers are not used because the greater cost of yttrium makes yttria barrier layers economically unfeasible.
A second solution (often implemented in addition to an alumina or silica barrier layer) has been to dose fluorescent lamps with excess liquid mercury. The result is that as mercury vapor is consumed, more liquid mercury vaporizes to sustain a dynamic equilibrium at mercury""s vapor pressure. However, increasing environmental concerns, as well as state and federal regulation of mercury, are requiring lamp manufacturers to dose fluorescent lamps with less mercury, not more, and excess liquid mercury is fast becoming a non-option.
There is a need in the art for a means of preventing, or substantially reducing, mercury consumption in fluorescent lamps. Such means preferably would eliminate the need for dosing a fluorescent lamp with substantial excess liquid mercury. Further, such means preferably would provide the benefits of yttria without necessitating a yttria barrier layer.
A mercury vapor discharge lamp is provided which has a light-transmissive glass envelope with an inner surface, means for providing a discharge, a barrier layer coated adjacent the inner surface of the glass envelope, a phosphor layer coated adjacent the inner surface of the barrier layer, and a fill gas of mercury and an inert gas sealed inside the envelope. The barrier comprises barrier layer substrate particles and 0.1-10 wt. % yttria. The barrier layer also has crystalline yttria particles uniformly dispersed throughout.
A mercury vapor discharge lamp is also provided which has a light-transmissive glass envelope with an inner surface, means for providing a discharge, a phosphor layer coated adjacent the inner surface of the glass envelope, and a fill gas of mercury and an inert gas sealed inside the envelope. The phosphor layer comprises phosphor particles and 0.001-10 wt. % yttria. The phosphor layer also has crystalline yttria particles uniformly dispersed throughout.
A method of providing a coating layer in a fluorescent lamp is also provided. The method comprises the steps of a) providing a suspension of 1-10 wt. % coating layer substrate particles in a suspension medium of deionized water; b) dissolving a yttrium salt in the suspension; c) adding hydrochloric acid to the suspension to bring the suspension to a pH of 3-6; d) applying the suspension to the inner surface of a glass envelope of a fluorescent lamp; e) drying the suspension coated on the inner surface of the glass envelope to provide a partially dried coating layer wherein the dissolved yttrium salt is at least partially recrystallized thereby; and f) baking the coating layer to dry the coating layer and to oxidize the recrystallized yttrium salt to yttria. The yttria is dispersed throughout the coating layer.